As is well known, clothing or leather articles and fashion accessories are very often enhanced with decorative elements that reproduce trademarks, logos, inscriptions and/or images. In the sector of fashion, and not only in it, widespread use is made of the technique of fastening to the clothing item or, generally, to the fabric or leather, strings of alphanumeric characters representing words, trademarks, logos or more in general alphanumeric codes. Ever more frequently, these decorations become primary elements identifying a company or a given product, consequently assuming significant importance. In view of their commercial weight, the aforesaid decorative elements have been progressively refined over time, until assuming particularly elegant and prestigious configurations. In particular, in addition to the conventional printed customizations, raised decorations have been devised, formed by individual metal inserts that are appropriately applied on the product to be characterized. These alphanumeric codes typically consist of a set of letters, numbers and symbols made, depending on requirements, of metal and/or of a plastic material and they can be fixed to the fabric according to several known methods. If the string consists of a single rigid piece, these known methods can comprise fixing the entire string on the fabric in a single operation by known techniques, such as punching or riveting. This mounting method has, first of all, the drawback of locally depriving the fabric of its elasticity and flexibility where the string was fixed, inasmuch as the string is a rigid element, in any case less flexible than the fabric on which it is applied. An alternative to this mounting method consists of fixing a character at a time if the string is not in a single piece, so as to make the characters mutually independent. In this way, the fabric maintains substantially unaltered its elasticity and flexibility. However, this latter solution is not free of drawbacks, which include the fact that fixing a character at a time on the fabric entails long mounting times with the possibility of losing the alignment between one character and another. In an additional alternative, methods have become widely used which entail the engagement of an insert comprising multiple characters obtained with a single casting and then eliminating the sprues and risers after fastening to the fabric/leather. The described system does not allow to completely eliminate the burrs and machining residues which, consequently, end up considerably deteriorating the aesthetic value of each insert. To eliminate or at least reduce the drawbacks briefly identified above, the Applicant in the past developed a system to apply the inserts concurrently as described in the Italian patent no. IT 0001399834. According to this technique, a single support is made which removably bears the string of inserts to be applied to the fabric. The single support has a plurality of seats counter-shaped to the individual insert to be applied. In this way, the single support allows to maintain a perfect alignment of the alphanumeric string during application. Once the inserts are fastened, the support itself is removed and discarded, leaving a customized product with each insert perfectly aligned and finished, but separate from the others. The subsequent European patent EP2434471 describes a similar technique, whereby it is possible to apply a plurality of separate inserts to a support. The prior art techniques described, today widely used in the market, including high fashion and luxury accessories, can be further improved, particularly from the viewpoint of customization. The Applicant has observed that the characteristic of the customization of objects, in particular for fashion accessories and even more in particular for high fashion accessories, in recent years has become an ever greater demand on the part of users, who are also little inclined to wait a long time before receiving the customized accessory. In particular, a need has been observed on the part of customers to obtain customizations with alphabetic or numeric characters of fashion accessories already acquired previously and directly in the sale store. The idea of the support that bears the inserts allows perfect alignment of the characters and their optimal positioning without imperfections, but it limits the customization of the product, in the sense that it is necessary a priori to decide the string to apply, which then the support reproduces. This entails, on one hand, the need to apply the same strings on many products in order to reduce the costs of technology, and on the other hand the impossibility of a customization after the finished product is placed on sale, limiting in fact the customer's freedom to decide which characters/strings to be affixed, for example concurrently with the purchase of the item. From additional considerations made, the Applicant has also observed that perfectly aligning the characters is not sufficient to assure the successful outcome of the operation of applying the characters. To assure the optimal aesthetic appearance, not only must the characters be mutually aligned, but the applied characters must be correctly positioned on the accessory, not limitedly so as to be axially centered and/or not oriented, for example, obliquely. The great variety of accessories on which these characters can be positioned, each distinguished by its own shape, makes perfect application of the aforesaid characters not trivial, in particular when they are applied by personnel who have not been perfectly trained for the activity. In addition, it is important to stress that the application procedure entails holing the accessory, for example a handbag (often, a high-value item), which, in the case of unsuccessful operations, could entail significant economic damage in addition to the purchaser's lack of satisfaction.